Alternative to itoa() for converting integer to string C++?

Question

I was wondering if there was an alternative to itoa() for converting an integer to a string because when I run it in visual Studio I get warnings, and when I try to build my program under Linux, I get a compilation error.

Answer 1

In C++11 you can use std::to_string:

#include <string>

std::string s = std::to_string(5);

If you're working with prior to C++11, you could use C++ streams:

#include <sstream>

int i = 5;
std::string s;
std::stringstream out;
out << i;
s = out.str();

Taken from http://notfaq.wordpress.com/2006/08/30/c-convert-int-to-string/

Answer 2

boost::lexical_cast works pretty well.

#include <boost/lexical_cast.hpp>
int main(int argc, char** argv) {
    std::string foo = boost::lexical_cast<std::string>(argc);
}

Answer 3

Archeology

itoa was a non-standard helper function designed to complement the atoi standard function, and probably hiding a sprintf (Most its features can be implemented in terms of sprintf): http://www.cplusplus.com/reference/clibrary/cstdlib/itoa.html

The C Way

Use sprintf. Or snprintf. Or whatever tool you find.

Despite the fact some functions are not in the standard, as rightly mentioned by "onebyone" in one of his comments, most compiler will offer you an alternative (e.g. Visual C++ has its own _snprintf you can typedef to snprintf if you need it).

The C++ way.

Use the C++ streams (in the current case std::stringstream (or even the deprecated std::strstream, as proposed by Herb Sutter in one of his books, because it's somewhat faster).

Conclusion

You're in C++, which means that you can choose the way you want it:

• The faster way (i.e. the C way), but you should be sure the code is a bottleneck in your application (premature optimizations are evil, etc.) and that your code is safely encapsulated to avoid risking buffer overruns.

• The safer way (i.e., the C++ way), if you know this part of the code is not critical, so better be sure this part of the code won't break at random moments because someone mistook a size or a pointer (which happens in real life, like... yesterday, on my computer, because someone thought it "cool" to use the faster way without really needing it).

Answer 4

Try sprintf():

char str[12];
int num = 3;
sprintf(str, "%d", num); // str now contains "3"

sprintf() is like printf() but outputs to a string.

Also, as Parappa mentioned in the comments, you might want to use snprintf() to stop a buffer overflow from occuring (where the number you're converting doesn't fit the size of your string.) It works like this:

snprintf(str, sizeof(str), "%d", num);

Answer 5

Behind the scenes, lexical_cast does this:

std::stringstream str;
str << myint;
std::string result;
str >> result;

If you don't want to "drag in" boost for this, then using the above is a good solution.

Answer 6

We can define our own iota function in c++ as:

string itoa(int a)
{
    string ss="";   //create empty string
    while(a)
    {
        int x=a%10;
        a/=10;
        char i='0';
        i=i+x;
        ss=i+ss;      //append new character at the front of the string!
    }
    return ss;
}

Don't forget to #include <string>.

Answer 7

С++11 finally resolves this providing std::to_string. Also boost::lexical_cast is handy tool for older compilers.

Answer 8

I use these templates

template <typename T> string toStr(T tmp)
{
    ostringstream out;
    out << tmp;
    return out.str();
}


template <typename T> T strTo(string tmp)
{
    T output;
    istringstream in(tmp);
    in >> output;
    return output;
}

Answer 9

Try Boost.Format or FastFormat, both high-quality C++ libraries:

int i = 10;
std::string result;

WIth Boost.Format

result = str(boost::format("%1%", i));

or FastFormat

fastformat::fmt(result, "{0}", i);
fastformat::write(result, i);

Obviously they both do a lot more than a simple conversion of a single integer

Answer 10

You can actually convert anything to a string with one cleverly written template function. This code example uses a loop to create subdirectories in a Win-32 system. The string concatenation operator, operator+, is used to concatenate a root with a suffix to generate directory names. The suffix is created by converting the loop control variable, i, to a C++ string, using the template function, and concatenating that with another string.

//Mark Renslow, Globe University, Minnesota School of Business, Utah Career College
//C++ instructor and Network Dean of Information Technology

#include <cstdlib>
#include <iostream>
#include <string>
#include <sstream> // string stream
#include <direct.h>

using namespace std;

string intToString(int x)
{
/**************************************/
/* This function is similar to itoa() */
/* "integer to alpha", a non-standard */
/* C language function. It takes an   */
/* integer as input and as output,    */
/* returns a C++ string.              */
/* itoa()  returned a C-string (null- */
/* terminated)                        */
/* This function is not needed because*/
/* the following template function    */
/* does it all                        */
/**************************************/   
       string r;
       stringstream s;

       s << x;
       r = s.str();

       return r;

}

template <class T>
string toString( T argument)
{
/**************************************/
/* This template shows the power of   */
/* C++ templates. This function will  */
/* convert anything to a string!      */
/* Precondition:                      */
/* operator<< is defined for type T    */
/**************************************/
       string r;
       stringstream s;

       s << argument;
       r = s.str();

       return r;

}

int main( )
{
    string s;

    cout << "What directory would you like me to make?";

    cin >> s;

    try
    {
      mkdir(s.c_str());
    }
    catch (exception& e) 
    {
      cerr << e.what( ) << endl;
    }

    chdir(s.c_str());

    //Using a loop and string concatenation to make several sub-directories
    for(int i = 0; i < 10; i++)
    {
        s = "Dir_";
        s = s + toString(i);
        mkdir(s.c_str());
    }
    system("PAUSE");
    return EXIT_SUCCESS;
}

Answer 11

Allocate a string of sufficient length, then use snprintf.

Answer 12

If you are interested in fast as well as safe integer to string conversion method and not limited to the standard library, I can recommend the format_int method from the {fmt} library:

fmt::format_int(42).str();   // convert to std::string
fmt::format_int(42).c_str(); // convert and get as a C string
                             // (mind the lifetime, same as std::string::c_str())

According to the integer to string conversion benchmarks from Boost Karma, this method several times faster than glibc's sprintf or std::stringstream. It is even faster than Boost Karma's own int_generator as was confirm by an independent benchmark.

Disclaimer: I'm the author of this library.

Answer 13

I wrote this thread-safe function some time ago, and am very happy with the results and feel the algorithm is lightweight and lean, with performance that is about 3X the standard MSVC _itoa() function.

Here's the link. Optimal Base-10 only itoa() function? Performance is at least 10X that of sprintf(). The benchmark is also the function's QA test, as follows.

start = clock();
for (int i = LONG_MIN; i < LONG_MAX; i++) {
    if (i != atoi(_i32toa(buff, (int32_t)i))) {
        printf("\nError for %i", i);
    }
    if (!i) printf("\nAt zero");
}
printf("\nElapsed time was %f milliseconds", (double)clock() - (double)(start));

There are some silly suggestions made about using the caller's storage that would leave the result floating somewhere in a buffer in the caller's address space. Ignore them. The code I listed works perfectly, as the benchmark/QA code demonstrates.

I believe this code is lean enough to use in an embedded environment. YMMV, of course.

Answer 14

The best answer, IMO, is the function provided here:

http://www.jb.man.ac.uk/~slowe/cpp/itoa.html

It mimics the non-ANSI function provided by many libs.

char* itoa(int value, char* result, int base);

It's also lightning fast and optimizes well under -O3, and the reason you're not using c++ string_format() ... or sprintf is that they are too slow, right?

Answer 15

int number = 123;

stringstream = s;

s << number;

cout << ss.str() << endl;

Answer 16

Note that all of the stringstream methods may involve locking around the use of the locale object for formatting. This may be something to be wary of if you're using this conversion from multiple threads...

See here for more. Convert a number to a string with specified length in C++

Answer 17

On Windows CE derived platforms, there are no iostreams by default. The way to go there is preferaby with the _itoa<> family, usually _itow<> (since most string stuff are Unicode there anyway).

Answer 18

Most of the above suggestions technically aren't C++, they're C solutions.

Look into the use of std::stringstream.